Typical inkjet printing mechanisms use cartridges, often called "pens," which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each cartridge has a printhead formed with very small nozzles through which the ink drops are fired. Most often, the printhead is held in a carriage that slides back and forth along a guide rod in a "reciprocating printhead" system, with the page being advanced in steps between each pass of the printhead. To print an image on paper media, for instance, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. Other printing systems, known as "page-wide array" printers, extend the printhead across the entire page in a stationary location and print as the media advances under the printhead. The particular ink ejection mechanism within either type of printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology.
For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is mounted within the printer chassis so the printhead can be maintained to promote printhead health. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. The wiping action is usually achieved through relative motion of the printhead and wiper, for instance by moving the printhead across the wiper, by moving the wiper across the printhead, or by moving both the printhead and the wiper.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper. Typically, these inks are supplied in a reservoir housed by the inkjet cartridge, so when the reservoir is emptied, the entire cartridge including the printhead is replaced in what is known as a "replaceable cartridge" system. Some cartridges are monochrome (single color), for instance, carrying only black ink, while other cartridges are multi-color, typically carrying cyan, magenta and yellow inks. Some printing mechanisms use four monochrome cartridges, while others use a black monochrome cartridge in combination with a tri-color cartridge.
Recently, an imaging cartridge system has been introduced by the Hewlett-Packard Company of Palo Alto, Calif., as the DeskJet.RTM. 693C model inkjet printer. This is a two-pen printer which uses a tri-color cartridge, carrying full dye-loads of cyan, magenta and yellow, and a black cartridge which may be replaced with a tri-color imaging cartridge. This imaging cartridge carries reduced dye-load concentrations of some colors, such as cyan and magenta, along with a full or partial dye-load concentration of black ink. The imaging cartridge allows the printer to produce more continuous tone changes, particularly flesh tones, so the resulting image has near-photographic quality, with very little graininess.
As the inkjet industry investigates new printhead designs, one tendency is toward using a "snapper" reservoir system where permanent or semi-permanent printheads are used and a reservoir carrying a fresh ink supply is snapped into place on the printhead. These snapper reservoirs are typically installed in reciprocating printers, which move both the printhead and the snapper reservoir back and forth across the media for printing. Another new design uses permanent or semi-permanent printheads in what is known in the industry as an "off-axis" printer. In an off-axis system, the printheads carry only a small ink supply reciprocally back and forth across the printzone, with this on-board supply being replenished through tubing that delivers ink from an "off-axis" main reservoir placed at a remote, stationary location within the printer. Rather than purchasing an entire new cartridge which includes a costly new printhead, the consumer buys only a new supply of ink or an "ink bag" for the main reservoir. Typically, the fresh ink supplies are sold individually by color, although in some implementations, a multi-color supply may be furnished.
From the discussion above, it is apparent that the vast majority of inkjet printing has been done on paper, although inkjet printing is often done on transparencies, foils, fabrics and other sheet-like media. It would be desirable to provide a new system which expands the concepts of inkjet printing to other uses, such as for printing images on hard or semi-flexible surfaces, and in particular, on human skin, such as for face-painting at carnivals and the like, in a manner that is both easy and economical to use. The matter of permanence, semi-permanence or temporariness of the printed image may be governed, at least in part, by the selection of the ink used to print the image, as well as the environment to which the printed image is exposed.